1. Field
The disclosure relates generally to electrical contactors.
2. Description of the Related Art
Low current electrical contactors may be found in various electrical systems, for example, motor starters. In a prior art low-current electrical contactor 100, an example of which is shown in FIG. 1, a moving contact bar 101 is positioned above a left stationary contact bar 102 and a right stationary contact bar 103. The three contact bars 101, 102, and 103 comprise respective contact discs 105A-B, 104A, and 104B. The contact discs are attached to the contact bars, and positioned so that the contact discs on the stationary contact bars 102 and 103 are directly opposed to corresponding contact discs on the moving contact bar 101. When the moving contact bar 101 is moved down toward the stationary contact bars 102 and 103, contact disc 105A approaches and touches contact disc 104A, and contact disc 105B approaches and touches contact disc 104B, closing a circuit between stationary contact bars 102 and 103 so that a current enters stationary contact bar 102 from current input 108 and flows through moving contact bar 101 to stationary contact bar 103, and exits stationary contact bar 103 via current output 109. The moving contact bar 101 is mechanically driven upwards and downwards by an actuating device 107, which transmits motion to the moving contact bar 101 through a spring 106.
As the moving contact bar 101 is mechanically driven toward the stationary contact bars 102 and 103, one pair of contact discs (e.g., 104A and 105A) may touch before the other pair (e.g., 104B and 105B), due to manufacturing tolerances. Therefore the linkage between the actuating device 107 and the moving contact bar 101 must have some flexibility, so that the contact bar 101 can pivot to cause the second pair of contact discs (e.g., 104B and 105B) to touch. The spring 106 may provide part of this flexibility.
The current is constricted as it flows through the points where the contact disc pairs 104A/105A and 104B/105B touch each other. This constriction generates a magnetic force proportional to the square of the current, which acts to drive the contact disc pairs 104A/105A and 104B/105B apart. This force may be referred to as the blow-apart force. During a fault event in electrical contactor 100, which may be caused by, for example, an external short circuit in the electrical system that contains electrical contactor 100, the currents in electrical contactor 100 may exceed a rated current level of the electrical contactor 100. The current is highly concentrated at each point of contact between the contact disc pairs, which may generate a correspondingly large blow-apart force at the point of contact. The spring 106 and the actuating device 107 must provide a closing force substantially greater than the total blow-apart force during a worst-case fault event. Otherwise, high currents may cause the metal that comprises the contact discs to melt at the point of contact, welding the contacts discs together.